1. Field of the Invention
The present invention relates to a heat conductive silicone grease composition which, even when filled with a large quantity of a heat conductive filler in order to provide superior thermal conductivity, still retains favorable fluidity and favorable handling properties, and also exhibits excellent durability and reliability under conditions of high temperature and high humidity.
2. Description of the Prior Art
Many electronic components generate heat during use, and in order to ensure satisfactory operation of these electronic components, the generated heat must be conducted away from the electronic components. Particularly in the case of integrated circuit elements such as the CPUs used in personal computers, the quantity of heat generated has continued to increase as the operating frequency has risen, meaning heat removal has become a significant problem.
Numerous methods have been proposed for removing this heat. Particularly in the case of electronic components that generate large quantities of heat, methods have been proposed in which the heat is dissipated by placing a heat conductive material such as a heat conductive grease or heat conductive sheet between the electronic component and another member such as a heat sink (see patent reference 1 and patent reference 2).
Known examples of this type of heat conductive material include heat-radiating greases that comprise a zinc oxide or alumina powder blended into a silicone oil base (see patent reference 3 and patent reference 4).
Moreover, in order to further improve the thermal conductivity, heat conductive materials that use aluminum nitride powder are also known. The patent reference 1 discloses a thixotropic heat conductive material that comprises a liquid organosilicone carrier, silica fiber, and one or more materials selected from amongst dendritic zinc oxide, aluminum nitride flakes, and boron nitride flakes. Patent reference 5 discloses a silicone grease composition obtained by blending a spherical hexagonal aluminum nitride powder with a specified particle size range into a specific organopolysiloxane. Patent reference 6 discloses a heat conductive silicone grease that uses a combination of a fine aluminum nitride powder with a small particle size and a coarse aluminum nitride powder with a large particle size. Patent reference 7 discloses a heat conductive silicone grease that uses a combination of an aluminum nitride powder and a zinc oxide powder. Patent reference 8 discloses a heat conductive grease composition that uses an aluminum nitride powder that has undergone surface treatment with an organosilane.
Aluminum nitride has a thermal conductivity of 70 to 270 W/(m·K), whereas diamond has an even higher thermal conductivity of 900 to 2,000 W/(m·K). Patent reference 9 discloses a heat conductive silicone composition that comprises a silicone resin, diamond, zinc oxide, and a dispersant.
Furthermore, metals also have a high thermal conductivity, and can be used in those situations where insulation of the electronic component is unnecessary. Patent reference 10 discloses a heat conductive grease composition obtained by mixing metallic aluminum powder with a base oil such as a silicone oil.
However, none of these heat conductive materials or heat conductive greases is able to satisfactorily cope with the quantity of heat generated by modern integrated circuit elements such as CPUs.
It is known from the theoretical equation of Maxwell and Bruggeman that the thermal conductivity of a material obtained by blending a heat conductive filler into a silicone oil is substantially independent of the thermal conductivity of the heat conductive filler if the volume fraction of the heat conductive filler is 0.6 or less. The thermal conductivity of the material only starts to be affected by the thermal conductivity of the heat conductive filler once the volume fraction of the filler exceeds 0.6. In other words, in order to raise the thermal conductivity of a heat conductive grease, the first important factor is to determine how to enable the grease to be filled with a large quantity of heat conductive filler. If such high-quantity filling is possible, then the next important factor is to determine how to enable the use of a filler with a high thermal conductivity. However, high-quantity filling causes a variety of problems, including a reduction in the fluidity of the heat conductive grease, and a deterioration in the workability of the grease, including the coating characteristics (such as the dispensing and screen printing characteristics), making practical application of the grease impossible. In addition, because the fluidity of the grease decreases, the grease becomes unable to fill minor indentations within the surface of the electronic component and/or heat sink, which causes an undesirable increase in the contact resistance.
With the aim of producing heat conductive materials with high-quantity filling and favorable fluidity, investigations have also been conducted into adding an alkoxy group-containing organopolysiloxane that treats the surface of the heat conductive filler, thereby causing a significant improvement in the dispersibility of the filler (see patent reference 11 and patent reference 12). However, these treatment agents degenerate via hydrolysis or the like under conditions of high temperature and high humidity, causing a deterioration in the performance of the heat conductive material.    [Patent Reference 1] EP 0 024 498 A1    [Patent Reference 2] JP 61-157587 A    [Patent Reference 3] JP 52-33272 B    [Patent Reference 4] GB 1 480 931 A    [Patent Reference 5] JP 2-153995 A    [Patent Reference 6] EP 0 382 188 A1    [Patent Reference 7] U.S. Pat. No. 5,981,641    [Patent Reference 8] U.S. Pat. No. 6,136,758    [Patent Reference 9] JP 2002-30217 A    [Patent Reference 10] US 2002/0018885 A1    [Patent Reference 11] US 2006/0135687 A1    [Patent Reference 12] JP 2005-162975 A